Government agencies of an increasing number of countries are applying exhaust emission control regulations to protect the environment. These regulations are being applied to all combustion engines including portable engines used in common equipment such as chain saws, lawn mowers and hedge trimmers. One means of reducing exhaust emissions even in small portable engines is to utilize four-stroke engines instead of the more conventional two-stroke engines. In the larger prior art conventional four cycle engines, an atmospheric vented crankcase includes an oil reservoir and oil pump or oil splash system for lubrication of the crankshaft, piston rings, and overhead combustion chamber intake and exhaust valves. Because small versions of the well-known larger conventional four-stroke engines are typically expensive to manufacture, they are generally not practical for smaller capacity applications. Hence, various alterations of the prior art conventional four-cycle engines have been developed for smaller and less demanding applications. One such alteration is known as a fuel mixture-lubricated four-stroke engine described in U.S. Pat. No. 6,199,532, to Haberlein et al, issued Mar. 13, 2001 and incorporated herein by reference.
In this fuel mixture-lubricated four-stroke engine, the lubricant or oil is pre-mixed with the combustion liquid fuel prior to flowing through a carburetor in much the same way as a two-stroke engine. The intake manifold is in direct communication with the valve cover and crankcase which is part of a sealed system. During the typically downward intake or induction stroke of this scavenging four-stroke engine, the fuel-air-lubricant mixture flows into the combustion chamber from the crankcase, valve chamber and the carburetor. During the upward compression and exhaust strokes, the fuel-air-lubricant mixture flows into the valve chamber and crankcase from the carburetor.
The primary source of lubrication for the crankshaft, intake and exhaust valves and related components is supplied to the valve cover chamber and crankcase during the compression and exhaust strokes. During these strokes, fuel-air-lubricant mixture generally flows directly into the valve cover chamber and crankcase while bypassing the combustion chamber altogether. The piston rings and cylinder walls are also lubricated primarily from below by this source of lubricant. Although not the primary source of lubricant, some lubrication of the piston rings from above or within the combustion chamber occurs when the fuel-air-lubricant mixture enters the combustion chamber during the intake stroke.
Also, in such engines during the downward power stroke, when the intake and exhaust valves are closed, the crankcase is overpressurized and requires venting. Such venting in prior art four-stroke engines is typically achieved by routing a vent passage to either the inlet or outlet side of the carburetor through a check valve which is normally closed and opens when the crankcase is pressurized at a predetermined superatmospheric pressure. Unfortunately, such venting is expensive to manufacture and thus not practical for smaller engine applications. Moreover, should the check valve fail in its open position or generally lack the necessary responsive reaction, spit back through the carburetor and oil drippage through the air filter can result. Other alternatives include routing the vent passage to the intake manifold without a check valve. Unfortunately, this leads to engine stalls during cold idle operation and rough engine idling during normal temperatures.